To change the "default" setting to the letter size paper, send one of the following commands to the printer as a text document. The easiest way to do this is from Athena. Place either single line command or the multi-line command in a text file and save it. Send it to the printer using the command, lpr filename.
%-12345X@PJL DEFAULT PAPER=LETTER
LPARM:PCL PITCH= 10.00